Coaxial-line to waveguide transition for horn antenna

ABSTRACT

THE INVENTION RELATES TO AN ANTENNA ASSEMBLY FOR MICROWAVE USAGE WHICH IS CHARACTERIZED BY A COAXIAL-LINE TO WAVEGUIDE TRANSITION. THE ANTENNA INCLUDES A RECTANGULAR WAVEGUIDE SECTION, A SECTORAL HORN, AND A TRANSFORMER FOR PROVIDING IMPEDANCE MATCHING BETWEEN THE WAVEGUIDE AND THE HORN. THE TRANSITION IS ACCOMPLISHED BY FEEDING TO THE END OF THE WAVEGUIDE AN E-PLANE LOOP, I.E., A LOOP PARALLEL TO THE ELECTRIC FIELD PLANE IN THE WAVEGUIDE. THE END FEEDING OF THE WAVEGUIDE RESULTS IN A MINIMUM VERTICAL DIMENSION OF THE OVERALL DEVICE WHEN THE COAXIAL LINE IS ATTACHED. THE SMALL VERTICAL DIMENSION IS DESIRABLE IN ORDER TO PERMIT STACKING OF A REQUIRED NUMBER OF ANTENNAS IN AN ARRAY HAVING LIMITED SPACE REQUIREMENTS BETWEEN ASSEMBLY INPUTS. THE INVENTION ALSO FEATURES A WIDE BAND RESPONSE.

Jan.12,1971 J.-E.Boms 3,555,553

COAXIAL-LINE TO WAVEGUIDE TRANSITION FOR HORN ANTENNA Filed Jan. 31, 1969 INVENTOR. J RRY E. BOYNS dd vuq/ ATTORNEYS United States Patent US. Cl. 343-776 5 Claims ABSTRACT OF THE DISCLOSURE The invention relates to an antenna assembly for microwave usage which is characterized by a coaxial-line to waveguide transition. The antenna includes a rectangular waveguide section, a sectoral horn, and a transformer for providing impedance matching between the waveguide and the horn. The transition is accomplished by feeding to the end of the waveguide an E-plane loop, i.e., a loop parallel to the electric field plane in the waveguide. The end feeding of the waveguide results in a minimum vertical dimension of the overall device when the coaxial line is attached. The small vertical dimension is desirable in order to permit stacking of a required number of antennas in an array having limited space requirements between assembly inputs. The invention also features a wide band response.

STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION In microwave transmission lines it is frequently necessary to change from waveguide to coaxial line. Proper matching of coaxial line to Waveguide requires that a transition be provided between the principal TEM mode of the coaxial line and the dominant TE mode in the waveguide. In most applications, for proper matching be tween a coaxial-line and a waveguide, a side-feed to the waveguide is used, i.e., a projecting metal exciting or absorbing antenna rod or other energy-translating apparatus is disposed approximately centrally in one of the sides of the waveguide.

Side-feeding of waveguides, however, presents difiiculties when it is desired to position several antennas in a stacked array. For example, if a horn-type radiator is to be used in -a circular array, the physical dimensions of the radiating element are determined by the desired beam width, antenna pattern, and the geometry of the circular array. The aperture and flare angle in the H-plane of the horn or open-ended waveguide portion of the element are determined according to mathematical equations for design parameters known to those skilled in the art. For example, J. D. Kraus in his book Antennas on page 374 describes design parameters for the optimum horn element and also gives the required mathematical equations for determining these design parameters.

If the requirements of the circular array application call for radiating elements mounted on a cylinder of large radius with a spacing as small as one-half wavelength between element centers the flare in the E-plane of the horn is limited by these requirements. Impedance matching consideration call for the largest possible flare while array geometry establishes the maximum amount of flare allowed as well as the maximum element length. Where a limited amount of space between the antenna element inputs is available, existing side-feeding techniques of the ice elements is not possible in the transition from coaxial-line to rectangular waveguide.

SUMMARY OF THE INVENTION The invention discloses an antenna assembly for use in antenna arrays which is characterized by a unique coaxial line to waveguide transition. The antenna includes a horn-type radiator and a transformer for matching the waveguide to the horn-type radiator .Transistion is provided between the principal TEM mode of the coaxialline and the dominant TE mode in the waveguide by feeding the end of the waveguide with an E-plane loop connected to the coaxial line by a miniature connector extending through the end of the waveguide. Such end feeding allows the waveguide to have a minimum vertical dimension which, in turn, permits stacking of the antennas in an array. The antenna assembly also features a very broadband response.

STATEMENTS OF THE OBJECTS OF THE INVENTION An object of the present invention is to provide an end-fed, coaxial-line to waveguide transition to permit stacking of atennas in arrays having limited space requirements.

Another object of the present invention is to provide an antenna assembly having a horn-type radiator which is end-fed and thus permits stacking of the antennas in an array.

Another object of the present invention is to proivde an antenna assembly in which miniature coaxial fittings are used to save space and weight.

Another object of the present invention is to provide an antenna assembly in which a waveguide having minimum dimensions is used to save space and weight.

Another object of the present invention is to provide an end-fed antenna assembly featuring a broadband response.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top-view of the coaxial-line to waveguide transition and radiating element of the present invention;

FIG. 2 is a side-view of the coaxial-line to waveguide transition and radiating element of the present invention;

FIG. 3 is a top view of a circular array of the radiating elements of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIGS. 1 and 2 are top and side views, respectively, of the coaxial line to waveguide transition and the radiating element of the present invention. The radiating element comprises a sectoral horn 15 of the open-ended type. The horn has a substantially rectangular cross-section, and flares smoothly from a throat or small end of the horn, to a mouth or aperture or large end 17 of the horn at its front. The aperture of the horn which has a dimension of approximately 3M2 in the H-plane and M2 in the E- plane, is electromagnetically opened to space to permit the born to receive electromagnetic waves of space or to radiate electromagnetic waves out into space. The H- plane is the plane at right angles to the electric vector and the plane parallel to the electric vector is the E-plane.

The principal or central axis xx of the horn extends between the small and the large ends of the radiating ele ment. The top and bottom sides 21 and 21' of the horn which are oppositely disposed are flared at an angle 0 with respect to the central axis. The other two oppositely disposed sides 20 and 20' are flared at an angle with respect to each other and with respect to the central axis. Flare angle q is considerably greater than flare angle 0.

At its throat, the sectoral horn 15 is shown connected to a rectangular waveguide consisting of three waveguide sections of approximately M4 each. The waveguide is closed at the end 22 and connected to the throat of horn at the opposite end. The miniature electrical connector 13 is centrally located and passes through closed end 22 of waveguide 10.

A metal exciting or absorbing loop 11 is located within waveguide 10 and is supported substantially parallel to the E-plane. The energy translating loop is connected with in the waveguide to miniature electrical connector 13.

Located Outside the waveguide is coaxial line 19 which terminates at one end in sending apparatus of the receiver or the transmitter type (not shown). Opposite end 23 of the coaxial line terminates on connector 13 on the outside of waveguide 10 to thereby provide a transition from coaxial line 19 to Waveguide 10.

A M4 transformer 12 is centrally located and supported within waveguide 10 to provide impedance matching between the waveguide and the horn element.

A dielectric radome 16 is located and supported within horn 15 at a distance of approximately M4 from mouth 17 of the horn to provide impedance matching between the antenna element and free space. The radome serves a dual purpose of improving the impedance match of the element to free space and of weatherproofing the element.

A gas inlet fitting 24 is provided inside of the horn element for the application of dry air to prevent moisture from condensing inside.

Located and supported on a top side of waveguide 10 is tuning slug 14 which is used in a well-known manner.

The radiating element of the present invention may be used for either transmission or reception of electromagnetic waves. In transmitting, the loop 11 first excites electromagnetic waves in the rectangular waveguide 10 Which are transmitted through the waveguide throat of the horn 15, and then propagated through the horn to the mouth 17 as horn waves. At the mouth, substantially all of this energy is radiated into free space as ordinary radio waves.

In receiving, a similar but reverse process takes place, i.e., the electromagnetic waves are received by horn 15, developing signals which are communicated to a receiving system (not shown).

The physical dimensions of the radiating element were determined by a desired beam width, antenna pattern, and the geometry of the desired cylindrical array as shown in FIG. 3. The aperture dimension and flare angle in the H-plane of the horn were determined according to well-known equations for design parameters for the optimum horn (e.g., J. D. Kraus Antennas, pp. 374-375) to meet the requirements of a bandwidth of 2.9 to 3.5 gc./s. with a VSWR of 1.5 :1 maximum over the band and with a beamwidth of at the center frequency of 3.2 gc./ s.

The flare in the E-plane of the horn was limited by the requirement of the circular array that the radiating elements 15 be mounted on top of each other along the circumference of a circular array support 30 with a spacing as small as M4 between element centers. Impedance matching considerations called for the largest possible flare while the array geometry established the maximum amount of flare allowed as well as the maximum element length.

Furthermore in a circular array, in order to obtain a particular antenna pattern with an acceptable side-lobe level, the correct number of antenna elements must be used and the distance between element centers Z must be less than to assure a single main beam and about M2 to maintain the proper side lobe level. Since the frequency bandwidth requirement, the distance between element centers Z, and the number of elements have been chosen, the radius 33 of the circular array can be determined.

Since the radius 33 of FIG. 3 is determined by the desired antenna pattern and side lobe level, the length L of the horn element itself must be considered. In order to obtain the proper beamwidth for the antenna pattern desired the horn element must be the proper length to provide the correct flare angle of the element.

Once the minimum length of the radiating element has been established, it will be found that a small amount of space Z will be left between input ends of the radiating elements. This space lwould not be sulficient to accommodate stacking of the required number of side-fed waveguides, as shown in dotted lines by reference numeral 31 in FIG. 3, since the radius is fixed by the beam desired. However, by using an end-fed transition from coaxial-line to waveguide, the requirements of a circular array can be satisfied as shown in FIG. 3.

The elements can be constructed'from brass; however, casting them out of aluminum would provide less weight for the array. Casting would also provide horns that would be similar and tolerances could be held Within limits. Furthermore, constructing the elements out of aluminum eliminates a dissimilar metal problem between the elements and the cylindrical ground. If the array has an aluminum ground and brass elements, it is necessary that measures be taken to reduce electrolysis.

It is possible to tune each element individually outside of the array with no further tuning being necessary after the array is assembled. If individual tuning is done, the tuning screw can be eliminated with a capacitive post of the right size being added to the inner wall of the transition. To eliminate the possibility of the elements becoming detuned after array assembly, the tuning screw can be soldered to the element to prevent detuning, and the screw can be machine-flushed to the outer wall of the element.

When the completed array has been assembled and tested, additional rings of elements might be added to provide further vertical coordinate scanning as well as horizontal coordinate scanning. T o avoid grating lobes in the pattern, a spacing of A or less between element centers vertically is required. If staggering of the elements is not possible in a multi-ring array, an element of smaller aperture can be used.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A coaxial-line to waveguide transition for use in stacked antenna arrays comprising:

(a) an open-ended sectoral horn adapted to propagate therein electromagnetic waves of wave length A within a bandwidth of 2.9 to 3.5 gc./s. and with a VSWR of 1.5 :1 maximum over said bandwidth, said horn having a pair of oppositely disposed sides flaring at an angle 6 with respect to each other and a second pair of oppositely disposed sides flaring at an angle with respect to each other, said angle 0 being substantially smaller than said angle said horn further having an aperture of approximately M2 in the E-plane and 3M2 in the H-plane;

(b) a rectangular waveguide having a length of approximately 3M4, said waveguide further having an open end connected to the input end of said horn and a closed end opposite said open end, said closed end having connected and passing therethrough miniature connector means;

(c) a coaxial-line located without said waveguide, said coaxial-line being terminated at one end thereof to said connector means and at the other end thereof to sending means;

(d) a metal loop located Within said waveguide and supported substantially parallel to said E-plane for exciting or receiving said electromagnetic Waves, said loop being connected to said connector means to thereby provide a transition from said coaxial-line to said waveguide;

(e) a M4 transformer located and supported within said waveguide at a distance of approximately M4 from said closed end of said waveguide, said transformer adapted to provide impedance matching between said Waveguide and said horn;

(f) a dielectric radome located and supported within said horn at a distance of approxmately M4 from the mouth of said horn to provide impedance matching between said antenna element and free space; and

(g) a gas inlet fitting connected to one of said pair of oppositely disposed flaring sides of said horn to prevent moisture from condensing within said horn.

2. The coaxial line to waveguide transition of claim 1 further comprising tuning screw means connected to said waveguide.

3. The coaxial-line to waveguide transition of claim 1 wherein said sending apparatus comprises transmitter means.

4. The coaxial-line to waveguide transition of claim 1 wherein said sending apparatus comprises receiver means.

5. A circular-array antenna comprising:

(a) a plurality of antenna elements having a length of approximately 3) consisting of:

(1) an open-ended sectoral horn adapted to propagate therein electromagnetic waves of wave length A within a 'bandwidth of 2.9 to 3.5 gc./s. and with a VSWR of 1.5:1 maximum over said bandwidth, said horn having a pair of oppositely disposed sides flaring at an angle 0 with respect to each other and a second pair of op positely disposed sides flaring at an angle with respect to each other, said angle 0 being substantially smaller than said angle said horn further having an aperture of approximately 7\/ 2 in the E-plane and DJ 2 in the H-plane;

(2) a rectangular waveguide having a length of approximately 3M 4, said waveguide further having an open end connected to the input end of said horn and a closed! end opposite said open end, said closed end having connected and passing therethrough miniature electrical connector means;

(3) a coaxial-line located without said waveguide, said coaxial-line being terminated at one end thereof to said connector means and at the other end thereof to sending means; and

(4) a metal loop located within said waveguide and supported substantially parallel to said E-plane for exciting or receiving said electromagnetic waves, said loop being connected to said connector means to thereby provide a transition from said coaxial-line to said waveguide;

(b) an antenna array support consisting of a thin-walled cylinder having a radius of approximately 13A, said cylinder adapted to support said antenna elements along the circumference of said cylinder in a stacked relationship to each other at a distance of approximately M2 between element centers.

References Cited UNITED STATES PATENTS 3,076,188 1/1963 Schneider 333--21 ELI LIEBERMAN, Primary Examiner U.S. Cl. X.R. 

